At present, high purity oxygen gases such as those having high purities of 99.5% or more are greatly demanded industrially as a welding gas or for medical use such as oxygen inhalation. For these purposes, oxygen gases having high purity of 99.5% or more obtained from cryogenic plants of air liquefaction are used exclusively.
The process for producing a high purity oxygen gas from air by using a cryogenic plant is a technique conventionally carried out on an industrial scale, and various improvements thereof have been also made. However, its economical efficiency is established only by the large scale production of several tens of tons per day or more, which can be accomplished only by consumption on a large scale. Accordingly, when a small amount of oxygen is used, the above high purity oxygen gas obtained from a cryogenic plant is transported in the form of liquid by using a tank lorry or packed in gas cylinders to supply it dividedly, although the costs thereof become extremely high.
On the other hand, processes for producing oxygen by PSA have been also developed.
For example, U.S. Pat. No. 4,190,424 discloses a process for producing oxygen by PSA using a carbon molecular sieve adsorption section and a zeolite molecular sieve adsorption section. However, in this process, a gas desorbed from the carbon molecular sieve adsorption section during a middle stage of desorption is used as a raw material gas of the zeolite molecular sieve adsorption section. Thus, the recovery of oxygen is limited because a large amount of oxygen is generated during an initial stage of desorption. Further, no pressure equalization between an adsorber wherein adsorption has been completed and an adsorber wherein desorption has been completed in the same section is effected in this process and therefore it is difficult to recover a gas having a high oxygen content from the carbon molecular sieve adsorption section from the beginning of the desorption operation. Furthermore, no rinsing of the zeolite molecular sieve adsorption section from its outlet which improves separation efficiency of N.sub.2 and O.sub.2 is effected in this process. On the other hand, a pretreatment for removing water and carbon dioxide is preferably employed in this process and therefore the cost of operation and equipment become high.
Japanese Patent Kokai No. 60-200805 corresponding to U.S. Pat. No. 4,566,881 discloses a process characterized by carrying out the regeneration of a first adsorption unit and a second adsorption unit alternately with the same vacuum pump. Thereby, in the second adsorption unit, the operation time is wasted and the regeneration time is deficient. Thus, the operation time in one cycle required for the second adsorption unit becomes twice that of the first adsorption unit. Therefore, the size of the second adsorption unit in comparison with that required for the same operation time in one cycle is twice as great, which results in higher equipment costs of equipment. Further, loss of pressure energy becomes larger because the total process system is composed of a pressure adsorption (several bar) step by a compressor and a vacuum regeneration step by a vacuum pump, and pressure recovery is not effected after pressure adsorption. For example, according to Example 1 thereof, the adsorption pressure of the first adsorption unit is raised to 7 kg/cm.sup.2 G (8 bar) by the compressor and therefore about 79% of N.sub.2 contained in air is also compressed simultaneously, which makes the process wasteful from the viewpoint of power saving. Further, a drying unit for pretreatment to remove water included in the air is provided in the system. Therefore, not only the costs of plant and equipment increase but also the operation steps become much more complicated.
The waste of time and larger power consumption lose are disadvantages of PSA for producing a high purity oxygen gas at a low cost.